The present invention relates to a method in which the wheel pressure in an electronically controlled motor vehicle brake system is determined, in which case the brake system includes inlet valves (3) and outlet valves (4) in the wheel circuits, the purpose of which is to control the pressure in the actuators (A . . . D) of the wheel brakes, with at least one of the valves, in particular all valves, in the area of the actuator being driven by way of one or more driver stage(s) which permits or permit adjusting the valve current. An electronically controlled calibration method of the outlet valve (4) is performed at least one single time, such method being used to calibrate one or more outlet valves in terms of their opening current behavior, and the outlet valve of one brake circuit is opened briefly by a corresponding electric actuation of the valve during a wheel pressure control operation for verification of the wheel pressure, and the current is measured in this case which flows exactly when the valve has opened (opening current), and the current measured is used as an indicator for determining the wheel pressure, taking into consideration the calibration values for the corresponding outlet valve which have been determined according to the calibration method.
Electronic brake control systems for ABS/ESP are fitted in up-to-date motor vehicles in many cases. The quality of the control of the brake pressure in an actuator of the wheel brake, among others, depends substantially on the knowledge about the current wheel pressure of the fluid which is used for pressure buildup. Therefore, pressure sensors are provided in each wheel circuit in addition to sensors which determine the pilot pressure. Besides, the quality of wheel pressure control can be improved further when the inlet valves are not only switching (digital valves) but are operated in analog or analogized fashion (A/D valves). The minimum requirement for a corresponding A/D actuation control is a pulse-width modulated current control which necessitates a corresponding current driver in the electronics of the brake control unit. For reasons of costs of manufacture, the increasing tendency is to substitute equivalent variables produced by pressure models for the pressure information received from the pressure sensors in the wheel circuits. It is this way possible to furnish a high quality of wheel control and save the additional costs for several pressure sensors in addition.
As stated above, it is known in the art to calculate the pressure that prevails in the actuator for the wheel (wheel pressure) using a so-called wheel model. As disclosed in the document WO 2004/101339 A1, this can be achieved in conventional brake control systems with inlet valves in a rather reliable fashion by balancing the pressure buildup times and the pressure reduction times on the basis of a self-learning algorithm, which monitors the current wheel control operations. In the event of more comfortable brake systems with A/D inlet valves, however, more pronounced differences will occur between the pressure defined by the model and the actual wheel pressure in some cases.
The invention at topic has for its object to still further improve a brake system of this type without wheel pressure sensors so that the quality of the brake control is still further enhanced, in particular in the ABS operation, with the result that the stopping distance is principally shortened.